Over the past several years, a great deal of emphasis has been placed on increasing the security of telecommunications systems, so that unauthorized persons cannot intercept and access voice, data, facsimile, video or other information not intended for them. Studies have shown that the commercial markets are well aware of the threats to their communications systems and have a clear sense of the business applications which are at risk because of potential breakdowns in security. The reasons for the heightened interest are many, including the increased use of cellular telephony in which a portion of the conversation is carried over the air and is thus more susceptible to attack, and the fact that other portions of the telecommunications network can be particularly susceptible to security breaches; i.e., wiring closets, junction boxes, manhole or telephone pole connections, facsimile machines, and especially cordless and cellular phones.
Despite the increased interest in secure communications, present security techniques have several limitations, including the general requirement that both the calling and called parties that desire to engage in a secure communication must have compatible security equipment that can send and receive encrypted signals using common handshaking protocols and encryption algorithms. If this is not the case, secure communications are normally not possible. Thus, at the present time, it is generally difficult for a calling party, such as a person using a cellular phone and thus desiring enhanced security, to communicate with a called party who is not properly equipped. One exception, described in U.S. Pat. 4,972,479 issued to R. W. Tobias, Jr. on Nov. 20, 1990, attempts to solve this problem, but does so in a complicated and uneconomical manner, involving a call diverter placed at the customer's home premises. In other situations, cellular telephone users can communicate with a mobile telephone switching office (MTSO) in an encrypted mode, and the MTSO completes the call to a destination in a clear (i.e., non-encrypted) mode. However, this type of arrangement is of limited usefulness, since access to the MTSO is limited to callers who are physically located in the area served by the MTSO; callers outside of this area cannot use the MTSO encryption facilities. Furthermore, the encryption arrangements currently used in the MTSO environment do not support conversion from one encryption algorithm to another, and do not support communications other than voice calls.
Likewise, it is generally not possible for a party receiving calls in an area in which he or she suspects possible compromise, to assure that his or her local phone link (e.g., from the terminating central office to the customer's premises) will receive encrypted information, in an instance when the call originator does not have a secure (i.e., an encryption-capable telephone. In addition, even if both calling and called parties do have secure telephones, they must both use the same protocols and algorithms in order to properly communicate with each other.